Conventionally, it is known such a four-wheel drive (referred to as “4WD” hereinafter) vehicle wherein an engaging force of a torque distribution clutch mechanism is adjustably controlled based upon a rotational difference between front wheels and rear wheels. FIG. 7 presents an example of a control map used in such a conventional 4WD vehicle, in which “T” of an axis of ordinates represents an engaging force of the torque distribution clutch mechanism, and “ΔN” of an axis of abscissas represents the rotational difference between the front wheels and the rear wheels.
Herein, in a case that the vehicle acceleratingly starts on a low friction road such as a snow road or crust road, the engaging force T of the torque distribution clutch mechanism is increased by using the control map shown by a one-dotted line A of FIG. 7. Therefore, steady acceleration and start can be performed in the 4WD vehicle.
However, in a case that the 4WD vehicle travels at a low speed at a large turning angle under condition that the engaging force of the torque distribution clutch mechanism is increased, the rotational difference between the front wheels and the rear wheel cannot be absorbed. As a result, this unabsorption of the rotational difference causes to generate a tight corner braking phenomenon (the vehicle difficulty turns so as to have activated the brakes). By this phenomenon, it is adopted such a map B that the gradient of the engaging force is easy in addition to the steep gradient of the map A. In such a situation, when a large steering angle is detected by a steering angle sensor and the like, it is judged to be on a tight corner state, so that the generation of the tight corner braking phenomenon can be prevented by using of the map B.
However, even if the 4WD vehicle travels on the snow road or the crust road that the 4WD vehicle easily slips, the engaging force of the torque distribution mechanism is diminished to change the control map to the map B when it is judged to be on the tight corner state. Therefore, the stability and traveling ability of the 4WD vehicle is decreased.